While x-rays have value for diagnosing the condition of a patient, ionizing X-ray radiation is itself harmful to living tissue. In recognition of this hazard, and with the hope of reducing radiation risks wherever possible, numerous organizations of radiation specialists have been developed throughout the world to report on radiation usage, certify radiation specialists, and make recommendations on radiation settings and procedures. These organizations include professional societies such as the Radiological Society of North America (RSNA) and European Society of Radiology (ESR), centers of learning such as American College of Radiology (ACR) and Royal College of Radiologists (RCR), agencies such as International Radiation Protection Association (IRPA) and International Atomic Energy Agency (IAEA), and commissions such as International Commission on Radiation Units and Measurements (ICRU) and National Council on Radiation Protection and Measurement (NCRP).
In the late 1970s, the International Commission on Radiological Protection (ICRP) proposed that a policy of ALARA (As Low As Reasonably Achievable) be adopted for radiological personnel and, more recently, for patients who undergo x-ray imaging. ALARA practice makes every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical. This effort is based on the awareness that any radiation exposure, no matter how small, carries with it a certain level of risk that is proportional to the level of exposure. The concept of ALARA has been adopted or supported by numerous professional organizations, but implementation of ALARA practice varies. Thus, actual exposure levels used for different types of imaging vary from region to region and even from site to site, based on practical factors such as equipment type and condition, user experience, pathology, personal preference, standard practices, regulatory requirements, and cultural influence.
While exposure reduction is a worthwhile goal, its implementation should not compromise the capabilities that radiological imaging systems offer to the diagnostician. Exposure level is itself one of the most influential factors in determining the diagnostic and image quality of a radiographic image. Incorrectly reducing X-ray exposure levels may result in poor quality images with reduced diagnostic value. Images produced with too little exposure can be characterized by problems such as excessive graininess and low contrast. These problems make such images more difficult to use and potentially compromise or imperil proper diagnosis. In some cases, exposure below a threshold level yields an image of inferior quality and limited utility; often, as a result, the patient must be re-imaged at a higher exposure level in order to generate a radiographic image of sufficient quality.
Using ALARA guidelines, manufacturers and users of x-ray equipment have expended considerable effort to develop both acquisition settings and procedural techniques that help to reduce exposure levels. For example, technique charts that provide recommended exposure settings for various conditions could be developed to meet the ALARA objective. These reduced settings may then be used for system tools that help to control dose levels, such as automatic exposure control (AEC) and anatomical programmed radiography (APR). Additionally, manufacturers and users of x-ray equipment have supported the ALARA concept by co-optimizing some or all of the imaging events such as image capture, image rendering, and image presentation.
There are times when current practices developed to support ALARA may need to be adjusted. Adjustment may be needed, for example, at introduction of a new source or detector technology, as a result of changed characteristics of the patient population such as patient age and size, with new support tools such as computer aided detection and computer aided diagnosis, and as a result of changing administrative, regulatory, or user strategy. Given an opportunity to view and assess displayed images representative of different exposure levels, the radiologist can then determine whether or not a lower dose image would be acceptable under various conditions. Implementation of such tools can help to reduce patient risk, without compromising image characteristics that relate to accurate diagnosis.
Different approaches to the problem of dose reduction have been proposed. For example, U.S. Pat. No. 7,280,635 entitled “Processes and Apparatus for Managing Low kVp Selection and Dose Reduction and Providing Increased Contrast Enhancement in Non-Destructive Imaging” to Toth describes an approach to defining a reduced dosage level for an imaging system based on an iterative method of obtaining actual image captures while changing driver parameters (e.g., kVp, mA, time). However, this approach requires numerous exposures of the test subject in order to gain an understanding of the preferred exposure level and would not, therefore, be desirable for anything other than real-time imaging such as fluoroscopy.
Another example, given in U.S. Pat. No. 5,396,531 entitled “Method of Achieving Reduced Dose X-Ray Fluoroscopy by Employing Statistical Estimation of Poisson Noise” to Hartley, describes a method for defining acquisition settings that optimize image quality while minimizing radiation dosage to the subject. The '531 patent addresses fluoroscopic imaging applications in which the diagnostician obtains real-time patient images using a fluoroscope. While low dose levels are typically used during fluoroscopy procedures, however, the length of a typical procedure often results in a relatively high exposure level to the patient. As with the Toth '635 disclosure, this approach requires multiple exposures of the patient in order to establish the preferred exposure level.
Simulation has been proposed as an alternate strategy for providing tools for defining or re-defining exposure levels that minimize patient exposure without compromising diagnostic image quality. In reduced-dose image simulation, an image that has already been acquired under a set of known, controlled conditions is used a basis image. From this basis, it is then possible to digitally generate new versions of the image as it would appear if it were acquired under various lower-dose conditions, without actually obtaining these additional acquisitions. Advantages of simulation over other approaches include: generation of an image without additional exposure to the patient, exploration of a range of exposure levels without risk of compromised diagnosis, obtaining images with identical positioning of the patient yet differing only in noise content, and evaluation of numerous patient types and pathologies.
There are a number of factors that affect exposure level in radiographic imaging, including the following: 1) energy distribution (keV) of the x-ray beam described by the maximum energy or accelerating voltage in kilovolts peak (kVp) and beam filtration; 2) tube current measured in milliamps (mA); 3) exposure time measured in seconds or fractions of a second; and 4) source to image distance (SID) measured in inches.
However, not all of these factors lend themselves to image simulation. Accelerating voltage is one example. Different anatomical structures such as bone, muscle, or fat, attenuate x-ray radiation in differing amounts as a function of the incident x-ray energy, keV. Over one range of energy levels specified by one accelerating voltage value, the attenuation of different types of tissue may vary significantly, while over another range specified by a different accelerating voltage value, very little attenuation difference may be perceived. Where the difference in attenuation is sufficient, incident radiation with proper intensity can generate an exposure at the imaging detector that allows differentiation between various anatomical components and, as a result, allows a radiologist to properly diagnose injury or illness from a radiographic image. Where the difference in attenuation is not sufficient, incident radiation may generate an exposure with little or no differentiation between anatomical components and the resulting image may be inadequate for the desired diagnosis. In a clinical setting, the accelerating voltage, and thus the energy distribution and, indirectly, radiation intensity, is chosen to maximize attenuation differences between the anatomical structures used in diagnosis. It is difficult to simulate a radiograph with a reduced exposure level due to modified accelerating voltage as it may require compensation of attenuation differences in anatomical components that were not discernible in the basis image. There is no way to accurately compensate for data that was never captured on the radiation-sensitive imaging plate that would have been present if a different accelerating voltage were used.
Other factors that do not readily lend themselves to simulation include patient positioning and x-ray source geometry. For instance, the radiation level depends on the distance from source to patient, but this also influences magnification and image sharpness in a complex fashion, which cannot be simulated from a two-dimensional projection measurement.
Other exposure factors, however, can be readily simulated, in particular the combination of tube current and exposure time. For instance, exposure time affects the amount of signal and noise levels in the image, conventionally expressed as the signal-to-noise ratio. By accurate modeling of the characteristic noise level as it changes with exposure time, it is possible to give the diagnostician some useful tools for determining the appropriate exposure time and thus potentially define new acquisition settings and procedural techniques related to exposure time that result in reduced radiation dose levels. Likewise, the magnitude of the x-ray tube current influences signal and noise in a linear manner, so that decreases in tube current for a fixed exposure time would decrease the signal-to-noise ratio in an computable manner. Thus, unlike accelerating voltage or patient positioning, exposure time and tube current are exposure factors that lend themselves to image simulation.
Numerous methods for generating low-dose radiographic images are provided in the literature. One example is disclosed in commonly assigned U.S. Pat. No. 7,480,365 entitled “Dose-Reduced Digital Medical Image Simulations” to Töpfer et al. Simulations carried out in this manner can be highly accurate. Other promising study results using images from cadavers were presented in a paper at the 2006 SPIE Medical Conference entitled “Preliminary Validation of a New Methodology for Estimating Dose Reduction Protocols in Neonatal Chest Computed Radiographs”, and in a 2006 RSNA Technical Exhibit entitled “Observer Performance in the Detection of Neonatal Pneumothorax: Use of a Stochastic Noise Generator to Simulate Reduced-Dose Computed Radiography” both by Steven Don, MD, et al.
While there are a number of proven simulation methods, at varying levels of maturity, however, there is a lack of tools for their systematic application. Characteristically, the task of planning and implementing a study for facilitating dose reduction decisions has been a daunting one, in terms of time, cost, and other factors, and efforts expended for this purpose have thus been narrowly limited to very specific types of images taken under a very limited range of conditions. Thus, it can be appreciated that there is a need for a utility that can help the diagnostician to systematically simulate and assess various imaging conditions in order to make accurate decisions for specifying appropriate dose levels for different types of radiographic images.